Epoxy resins have been widely used in paints, insulating materials for electric and electronics fields, adhesives and the like, because the hardened epoxy resins has a superior quality in mechanical properties, electric properties, thermal properties, chemical resistance, adhesiveness and the like. Epoxy resin compositions in general use at this time are a so-called two-liquid type, for which two-liquids, an epoxy resin and a hardener, are mixed before use.
The two-liquid epoxy resin composition can be hardened at room temperature but the storage and handling are complicated because the epoxy resin and the harder have to be stored separately, and both have to be weighed and mixed as necessary before use. Furthermore, since the usable time is limited, it is impossible to pre-mix them in a large quantity which creates a problem of frequent mixing and low efficiency.
To solve such problems of the two-liquid epoxy resin combination, several one-liquid epoxy resin compositions have been proposed up until now. For example, a latent hardener such as dicyandiamide, BF3-amine complex, amine salts, denatured imidazole compounds has been mixed with epoxy resins.
However, these latent hardeners having good storage stability have low curability requiring high temperature or long time for curing, while those having high curability have low storage stability requiring, for example, storage at a low temperature such as −20° C. For example, although the storage stability of the combination at normal temperature in 6 months or longer for dicyandiamide, a curing temperature of 170° C. or higher is required, and when a curing promoting agent is used together to decrease the curing temperature, curing is possible at 130° C. but the storage stability at room temperature is not sufficient, and the composition must be stored at a low temperature. Thus a composition having both high curability and superior storage stability has been strongly desired. Further, when products of film shape or an epoxy resin impregnated base are prepared, these products are often mixtures containing solvents and reactive diluents, and conventional latent hardeners used for such mixtures have extremely low storage stability. It is also impossible to prepare one-liquid varnish in which hardeners are mixed with solvents, and varnishes essentially need to be two-liquid products and an improvement in this point has been desired.
To respond these requirements, so-called microcapsulated hardeners have been proposed which contain cores of amine hardeners covered with specific shells, succeeding to obtain both low temperature curability and storage stability to some extent (for example see Patent Document 1). In this Patent Document 1, a master batch type hardener for one-liquid epoxy resin compositions, which contain a hardener composed of a core of a specific amine hardener and a shell of a reaction product of an isocyanate compound or a reaction product of the aforementioned amine compound and an epoxy resin, is disclosed.
However, in recent years, especially in the field of electronics apparatus, further improvement in curability without impairing storage stability is demanded for one-liquid epoxy resin compositions, which are used as one of connecting materials, in order to match high density electric circuit and to improve connection reliability as well as to use materials having low heat resistance for making mobile devices lighter, or to improve productivity great deal.
To satisfy such demands, latent hardeners have been proposed in which a hardener is covered with a film obtained by a reaction of a specific component (for example, see Patent Document 2).
Patent Document 1: JP-A-1-70523
Patent Document 2: JP-A-2005-344046